WP uses many flags in the code. Together we can document what we know.
WP_USE_MULTIPLE_DB is used when you need to run wpmu off more than one database. It allows the administrator to spread users across several databases.
It's not fully implemented so unless you have more than 10K users or are running on a dedicated machine its best not to worry with it.
Other than wp.com, very few people run wpmu off of more than one db.